Moving voice coil electro-acoustic converter with laminated magnetically anisotropic poles

ABSTRACT

An improvement is an electro-acoustic converter having a magnetic circuit with an air gap therein, the improvement including a magnetically anisotropic material in the magnetic circuit. The axes of the anisotropic material are aligned to eliminate flux caused by a voice coil.

United States Patent Yamamuro 1 1*Nov. 25, 1975 15 MOVING VOICE COIL 3,665,352 5/1972 Dietrich e131 179/119 ELECTRO ACOUSTIC CONVERTER WITH 3,783,311 l/l974 Sato et al. 1 1 v 179/1155 R 1830,9861 8/1974 Yamamuro v, 179/1155 R LAMINATED MAGNETICALLY ANISOTROPIC POLES [75] Inventor: lsao Yamamuro, Tokorozawa.

Japan [73] Assignee: Pioneer Electronic Corporation, Tokyo, Japan 1 Notice: The portion of the term of this patent subsequent to Aug 20 1991, has been disclaimed.

[22] Filed: Dec. 18, 1972 [21] Appl. No: 315,962

[30] Foreign Application Priority Data Dec. 17, 1971 Japan 46-10251? [52} US. Cl. .1 l79/ll5.5 R; 179/119 R [51] Int. Cl. H04R 9/02 [58] Field of Search 179/1155 R, 115.5 VC, 179/117 119 R; 335/231, 236, 296, 301 302, 304

[56] References Cited UNITED STATES PATENTS 3,632,904 1/1972 Mauz 179/1155 R FOREIGN PATENTS OR APPLICATIONS 923,537 4/1963 United Kingdom 335/302 OTHER PUBLICATIONS Brailsford, Magnetic Materials, 1960 Methuen & Co., Chaps. 4 & 5, PP- 84-136.

Primary Examinerl(athleen H. Claffy Assistant ExaminerGeorge G. Stellar Attorney Agent, or Firm-Sughrue, Rothwell, Mion Zinn & Macpeak ABSTRACT 1 Claim, 9 Drawing Figures US. Patent FIG. lo

PRIOR ART Nov. 25, 1975 Sheet 1 of 3 FIG. lb

PRIOR ART C Cl US. Patent Nov. 25, 1975 Sheet 2 of3 3,922,501

FIG. 4

50 I00 200 500 I000 2000 5000 |0000 FREQUENCY (Hz) 0 O O O O 0 IO IO N US. Patent Nov. 25, 1975 Sheet 3 of3 3,922,501

1| II v 0ZwDOwmm 089 oooomw fip m oo9 8m 8 0 09 on 8 MOVING VOICE COIL ELECTRO-ACOUSTIC CONVERTER WITH LAMINATED MAGNETICALLY ANISOTROPIC POLES CROSS-REFERENCE TO RELATED APPLICATIONS This application is related in subject matter to copending applications Ser. No. 315,963 and 315,970 filed Dec. 18, 1972, by Isao Yamamuro and assigned to a common assignee with the present application. Application Ser. No. 315,963 has issued as U.S. Pat. No. 3,830,986. Application Ser. No. 315,970 has issued as U.S. Pat. No. 3,867,587.

BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to a magnetic circuit for an electro-acoustic converter such as a speaker or a microphone.

2. Description of the Prior Art Normal magnetic material has a magnetic hysteresis loop characteristic and the relation of magnetizing force and magnetic flux density is non-linear. Therefore, when magnetic flux of a voice coil passes through a center pole and plate near an air gap, the magnetic reaction in the voice coil results in a distorted output signal from the electro-acoustic converter such as a speaker or a microphone.

Accordingly, various methods eliminating the distortion have been attempted such as methods of covering a center pole a with a copper cap b as shown in FIG. la, putting in a copper ring as shown in FIG. lb, or mak ing a depression on an upper surface of a center pole a and plating the depression with copper d as shown in FIG. 1c. The above-mentioned methods shown in FIGS. la and lb, however, do not only require a higher level of manufacturing technique than a conventional normal magnetic circuit for making the air gap a predetermined size, but also results in decreasing the magnetic flux density within the air gap due to the substantial increase in the air gap width. When using the copper ring 0, the copper ring disturbs the magnetic flux distribution in the air gap thus disturbing the output signal. In the last case shown in FIG. 10, it is difficult to uniformly plate with sufficient thickness.

SUMMARY OF THE INVENTION The present invention has taken notice of the aforementioned problems and eliminates the distortion caused by the magnetic circuit. The primary object of the present invention is to provide a magnetic circuit for an electro-acoustic converting device characterized in that a portion or all of the magnetic circuit elements defining an air gap are formed of a magnetic anisotropic material, thereby increasing the magnetic resis tance to magnetic flux caused by a voice coil and eliminating the reproduction distortion caused by a magnetic non-linearity of a magnetic circuit.

BRIEF DESCRIPTION OF THE DRAWINGS FIGS. -10 are views in vertical section, of conventional magnetic circuit portions for an electro-acoustic converter.

FIG. 2 is a view in vertical section, of one embodiment of a magnetic circuit in accordance with the present invention.

FIG. 3 is an enlarged fragmentary sectional view schematically illustrating directions of magnetic flux.

FIG. 4 is a diagram illustrating the current distortion characteristic of a speaker using a conventional magnetic circuit.

FIG. 5 is a diagram illustrating the current distortion characteristics of a speaker using the magnetic circuit according to the present invention.

FIG. 6 is a view in vertical section, of another embodiment of the present invention.

FIG. 7 is an enlarged sectional view of the additional embodiment.

DESCRIPTION OF THE PREFERRED EMBODIMENT Explaining one embodiment of the present invention in accordance with the drawings, 1 is a yoke, 2 is a center pole fixed in the center of the yoke l, 3 is a ringshaped magnet, 4 is a plate fixed on the magnet 3, 5 is a voice coil cylinder, and 6 is a voice coil.

In the present embodiment, the upper outside por tion of the pole 2 and the inside portion of the plate 4 are respectively constructed with parts 7 and 8 which are magnetically anisotropic, such as a two directional silicon steel. The easy magnetizing axis and hard mag netizing axis of the parts 7 and 8 are respectively directed at right angles and parallel to the voice coil 6.

Therefore, the magnetic flux out of the magnet 3 returns to the same magnet 3 by way of the plate 4, the easy magnetizing axis of the magnetically anisotropic part 8, the air gap, the easy magnetizing axis ofthe magnetically anisotropic part 7, the center pole 5, and the yoke 1. Namely, the magnetic flux for driving the speaker passes the air gap through the same route as in the conventional magnetic circuit. The total magnetic resistance of the magnetic circuit is slightly increased as compared with that of the conventional magnetic circuit, because the magnetic flux for driving the voice coil passes through the easy magnetizing axis of the magnetically anisotropic parts 7 and 8.

The magnetic flux caused by the voice coil 6 follows the path shown by dotted lines in FIG. 3, that is, the magnetic flux completes closed loops which pass along both the hard magnetizing axes of the magnetically anisotropic parts 7 and 8. However, the magnetic flux passing the magnetic anisotropic parts is limited because of the high magnetic resistance and the magnetic saturation brought about with the small magnetic flux. Consequently, the reproduction distortion due to the magnetic non-linearity of the magnetic circuit elements diminishes greatly because of the weak mutual relation of the magnetic flux of the voice coil 6 and the magnetic circuit parts. Further, since the electrical resistance of the two directional silicon steel is higher than normal iron, the silicon steel is useful for diminishing eddy current power loss caused by the magnetic flux of the voice coil in the magnetic circuit elements.

FIG. 4 is a diagram illustrating current distortion characteristics of a speaker using a conventional magnetic circuit. FIG. 5 is a diagram illustrating current distortion characteristics of the speaker using a magnetic circuit according to the present invention, in which the upper outside portion of the center pole 2 and the inside portion of the plate 4 are constructed with the two directional silicon steel which is magnetically anisotropic. Comparing these characteristic diagrams, it is seen that the second harmonic distortion A and the third harmonic distortion B in case of the present invention speaker diminishes greatly in comparison with the second and third harmonic distortion curves A and B of the conventional speaker. The characteristic curve between I-ll of the second harmonic distortion curve A shown in FIG. shows a filter characteristic of a distortion detector, the true distortion component is scarcely included between the HI.

In the aforementioned present embodiment, a por tion of the elements of the magnetic circuit are constructed with the magnetically anisotropic material. The whole of the plate 4 and the upper part of the center pole 2 may be composed of the magnetically anisotropic material as shown in FIG. 6, or may be composed of parts which have magnetically anisotropic characteristics, which laminate sheets 9 of the normal magnetic material or the magnetically anisotropic material. When using the laminated parts as shown in FIG. 7, an insulated or conductive lamination 10 may be positioned between sheets of the normal magnetic material. When the insulated lamination 10 is used, the eddy current power loss decreases and the magnetic flux acts to good advantage, and thus an electro-acoustic converter having a high conversion efficiency may be manufactured without difficulty. When the lamination 10 is conductive, the conductive laminations act as short rings, and thus the distortion is reduced more than with only the magnetically anisotropic material.

As described above, the present invention is characterized in that a portion or all of the magnetic circuit sections which define the boundaries of an air gap have portions which are magnetically anisotropic, namely 4 the portion having high magnetic resistance against a magnetic flux of a voice coil and low magnetic resistance against a magnetic flux for driving a speaker. Thereby, the above-mentioned problems have been corrected and the objects of the present invention are effected.

In addition, the present invention may be applied to other electro-acoustic converters such as a microphone.

While the invention has been particularly shown and described with reference to the preferred embodiment thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention.

What is claimed is:

1. In an electro-acoustic converter comprising:

a. a center pole;

b. voice coil means surrounding said center pole;

c. magnetic means surrounding said center pole and i said voice coil means; and

d. plate means fixed to said magnetic means, said plate means and said center pole defining an air gap accepting said voice coil; the improvement comprising e. wherein all of said plate means and that portion of said center pole defining said air gap consist of magnetically anisotropic sheets laminating magnetically anisotropic sheets.

i t t I! 

1. In an electro-acoustic converter comprising: a. a center pole; b. voice coil means surrounding said center pole; c. magnetic means surrounding said center pole and said voice coil means; and d. plate means fixed to said magnetic means, said plate means and said center pole defining an air gap accepting said voice coil; the improvement comprising e. wherein all of said plate means and that portion of said center pole defining said air gap consist of magnetically anisotropic sheets laminating magnetically anisotropic sheets. 